In a wet-type electrophotographic printer using a liquid developer, inasmuch as its toner density impacts the printing quality (image density) of a printed matter, it has hitherto been practical to attempt to control the toner density of a liquid developer so as to be kept at a selected value.
For example, as disclosed in JP 2009-163124 A, a toner density of a liquid developer supplied to a developing machine from a main tank is detected utilizing the fact that the velocity of ultrasonic waves propagating in the liquid developer is varied with the toner density. The toner density of a liquid developer is controlled so that toner densities so detected become identical to a given value (JP 2009-163124 A).
In detecting a toner density as mentioned above, it has also been known that air bubbles if contained in a liquid developer vary the propagation velocity of ultrasonic waves therethrough, leading to a reduction of the detection accuracy of a toner density (see JP 2008-14930 A).
In a toner density detecting apparatus as disclosed in JP 2008-14930 A cited above, only a response time from transmission to reception of one pulse of the ultrasonic waves from an ultrasonic transmitter, in which a propagation velocity per one pulse of the ultrasonic waves is within a selected range of velocities, is used in an attempt to improve the detection accuracy of the toner density if air bubbles are contained in the liquid developer.
In recent years, a deaerator for separating and thereby removing air dissolved in a liquid has been proposed in which a hollow fiber membrane (gas permeable membrane) is utilized.
The hollow fiber membrane has a fiber wall in the form of a straw which is formed with ultrafine holes and in nature is permeated only with dissolved gas of the molecular level existing in a liquid, the fiber wall thereby separating dissolved air and removing the same from the liquid.
For example, as the deaerator for removing a gas (air) dissolved in an ink in an ink jet recording machine there has been proposed one in which numbers of hollow fiber membranes as mentioned above are disposed in a case and passed by the ink through their insides while the inside of the case is depressurized, thereby removing the gas (air) from the ink passing through the inside of the hollow fiber membranes (see JP 2006-281532 A).
While a toner density detecting apparatus as disclosed in JP 2008-14930 A cited above allows a toner density in a liquid developer, even if it contains air bubbles, to be detected with a high degree of precision, to the extent that air bubbles are contained in the liquid developer there exists the possibility that a liquid developer circulating system for detecting the toner density and the printing quality of prints are thereby adversely affected in any way; hence potentially rising the need to further take measures to meet such possibilities.
Also, demanded with a desired increase in printing speed of an electrophotographic printer, an increase in amount of consumption of toner per unit time and an increase in rate of supply of a liquid developer onto a developing machine tend to further grow adverse influences of the existence of air bubbles on the printing quality of prints
Whereupon, utilizing a deaerator as disclosed in JP 2006-281532 A which uses a hollow fiber membrane for removing gases in an ink in an ink jet recording machine, the present inventors attempted to remove air dissolved in a liquid developer and causing air bubbles in the liquid developer, but without satisfactory results. In an attempt to detect a toner density of the liquid developer using ultrasonic waves, toner densities could not be detected accurately due to such air bubbles.
Causes of this have been found to be as follows:
A first cause: Having a viscosity higher than that of an ink for an ink jet recording machine, a liquid developer for use in a wet-type electrophotographic printer needs to be passed through the inside (inner diameter portion) of a hollow fiber membrane constituting an aforesaid deaerator and having a diameter of several millimeter, under an elevated pressure to allow the high viscosity liquid developer to flow therethrough at a desired rate of flow. However, for fear the liquid developer under elevated pressure may cause the fiber forming the hollow fiber membrane to peel off, the liquid developer cannot be passed to flow at the desired rate of flow.
A second cause: The rate of flow of the liquid developer of high viscosity when passing to flow through the inside of the hollow fiber membrane is high as desired only in a central region of the inside, but is lowered in its outer region adjacent to the inner peripheral surface (inner wall surface) of the hollow fiber membrane. As a result, air dissolved in the liquid developer becomes hard to flow out through the wall of the hollow fiber membrane, and the efficiency of removal of the dissolved air may become extremely deteriorated.
Made to solve problems as mentioned above, the present invention has for its object to provide an apparatus for detecting a toner density of a liquid developer for an electrophotographic printer whereby in detecting a toner density utilizing the fact that the propagation velocity of ultrasonic waves propagating through the liquid developer changes with a change in toner density, it is made possible to detect a toner density of the liquid developer with a high degree of precision.